1. Field of the Invention
The present invention relates in general to the field of electronic devices and, more particularly, to an antenna system for an electronic device having a range of antenna positions to optimize antenna performance without exposing a user to excessive radiation.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Many electronic devices are designed to communicate via a wireless transmission protocol. Most portable computers are now purchased with built in wireless networking capability using communications protocols such as, for example, 802.11a, b, or g wireless local area networking (WLAN), Bluetooth, and wireless wide area networking (WWAN). Electronic devices that connect to a WWAN, such as portable computers that are able to connect to an existing mobile broadband service, are gaining in popularity. Connection with such a network requires the computer to transmit radio frequency (RF) signals and receive transmissions of RF signals from other similarly capable devices such as wireless access points, wireless routers, and other electronic devices. As such, electronic devices may conceal a multitude of antennas to execute transmissions via these various radio frequency protocols.
An antenna functions by transmitting and receiving RF waves, or, RF electromagnetic energy. As such, a functioning antenna consumes energy and emits it in the forms of heat and emitted RF energy. In other words, energy that is input to the antenna and not radiated as electromagnetic energy is dissipated as heat. The percentage of energy consumed by the antenna that is dissipated as RF electromagnetic energy is known as the antenna's efficiency. Antenna efficiency may be an important characteristic of an electronic device because it serves as a measure of how much energy is required to power the antenna. Antenna efficiency may also be used to indicate signal strength in cases where a transceiver draws a fixed amount of power. Since power consumption may affect device performance, it is normally desirable to have an antenna that is somewhat efficient. One problem with many typical antenna designs is that they do not extend beyond the body of the electronic device. As such, the structure of the device may inhibit RF radiation, and thereby cause the antenna to operate inefficiently by reducing the proportion of antenna power that is emitted as RF radiation.
Another important aspect of antenna operation is the effect of RF radiation on the human body. When a user sits in close proximity to an antenna, their body may absorb a portion of the electromagnetic energy that is emitted by the antenna. This can be dangerous in some cases because exposure to RF electromagnetic radiation may cause biological tissue to heat rapidly. As a result, regulatory bodies, such as the Federal Communications Commission (FCC) in the United States and the European Commission (EC) in the European Union, promulgate safety regulations that limit the extent to which a device may expose a user's body to radiated electromagnetic energy. These regulations function by limiting the specific absorption rate associated with a device. Here, specific absorption rate (SAR) is a measure of the amount of RF energy that an amount of biological tissues absorbs when exposed to an RF electromagnetic field. SAR, normally expressed in watts per kilogram (W/kg) or milli-watts per gram (mW/g), is limited by regulatory bodies for many transmitting electronic devices. The FCC and EC require that all radio transmitting devices pass a specific absorption rate requirement.
As noted above, SAR is generally measured in terms of the amount of energy that will be absorbed by a mass of tissue. However, SAR requirements may vary based on the intended use of the device. For instance, a device that is meant to be in contact with a user's head may be subject to a more stringent SAR requirement because the head is a sensitive part of the body. Thus, SAR limits may be expressed in several ways. For example, in the United States a spatial average limitation functions to limit the amount of energy absorbed over the entire body of the user over a period of time to 0.08 W/kg; a spatial peak limitation functions to limit the amount of energy absorbed by any one particular (cube shaped) gram of tissue averaged over a period of time to 1.6 W/kg; and a spatial peak limitation applied to less sensitive parts of the body functions to limit the amount of energy that may be absorbed over a period of time to 4 W/kg.
SAR measurements will decrease exponentially over distance. This means that a device that employs an internal antenna, such as a laptop computer with a folding liquid crystal display, may more easily meet SAR requirements than a device with an external antenna by creating a cushion space between the antenna and the user. Conversely, an engineer may encounter some difficulty when seeking to maintain SAR compliance while designing an electronic device with a flat profile, such as a tablet computer or personal digital assistant (PDA) because there is no component of the device that can be assumed to protrude away from the body of the user. These types of devices are also poorly suited for incorporating an internal antenna because the body of the device will limit the efficiency of the antenna. As a result of these concerns along with the increasing popularity of flat profile electronic devices that are able to communicate wirelessly, integrated external antennas are becoming increasingly popular. Unfortunately, adding an external antenna to these types of products may require compromises in the mechanical or industrial design of the product because antenna design considerations may counteract each other. For instance, while it is desirable to keep the antenna at optimal efficiency, doing so may be difficult without compromising the aesthetic appearance of the product, inhibiting the utility of the device, or making the device less robust.
Further, while it may be easy to accomplish higher antenna efficiency with an external design, external antennas are generally more fragile, obtuse in appearance, and difficult or expensive to replace when they become damaged. Also, external antennae must be kept away from the body of the user so that the device will comply with SAR requirements. Traditional antenna designs, including telescoping antennae and flexible antennae fall short in addressing these shortcomings. Telescoping antennae, which are commonly used in external antenna designs, are typically made from a thin wall metal which makes them fragile and easy to damage. Flexible antennae, such as an antenna composed of a flexible radiating element and coating, are prone to surface coating cracking and fatigue failure. By failing, either of these antennae designs may cause costly repairs for an end user, and may ultimately require a manufacturer to expend time and resources providing technical support to help an end user replace their antenna.
Accordingly, it would be desirable to create an external antenna design that is optimized in terms of efficiency, while also achieving pleasing aesthetics and a robust design. It is also imperative that the antenna design can be implemented in a way that will offer compliance with FCC and EC SAR level limits.